-fsanitize=* options makes GCC less smart than usual and increase number
of 'maybe-uninitialized' false-positives. So this patch does two things:
* Add -Wno-maybe-uninitialized to CFLAGS_UBSAN which will disable all
such warnings for instrumented files.
* Remove CONFIG_UBSAN_SANITIZE_ALL from all[yes|mod]config builds. So
the all[yes|mod]config build goes without -fsanitize=* and still with
-Wmaybe-uninitialized.